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For sensitive and reproducible surface-enhanced Raman scattering (SERS)-based trace detection, a SERS platform with high concentration of homogeneously distributed hotspots is needed. Herein, we report a facile monolayer assembly of multispiked gold nanoparticles (MSGNPs) of various sizes over a large area. The assembly was achieved simply by tuning the concentration of MSGNPs in a suspension and subsequently drying it on a substrate under ambient conditions. A monolayer assembly for MSGNP sizes ranging from 150 to 640 nm has been demonstrated. The MSGNP assembly showed excellent sensitivity and uniformity of SERS-based trace detection, owing to the presence of high concentration of uniformly distributed interparticle and intraparticle hotspots. In particular, detection of 10 fM of a toxic dye, crystal violet, has been demonstrated. Moreover, ammonium nitrate, which is a commonly used material for homemade explosives, was detected at 1 mu M level. The MSGNP-based SERS platforms have great potential for applications in sensitive and reliable toxin and explosive detection.

Conference Name: Plasmonics: Design, Materials, Fabrication, Characterization, and Applications XIX; 1179713 (2021) Date of Conference: 2021 SERS has become a highly preferred method with the effect of advances in instrument technology and potential for application to many areas from medicine to art conservation for the detection of trace-level environmental and biological analytes. However, the effects of substrate variation and the details of the enhancement mechanism still of concern even much progress has been achieved in recent years. Therefore, it is important to control the size and shape of the materials for maximize the enhancement factor and ensure the reproducibility of the substrates by considering SERS uncertainty principle. In this study, we present novel SERS substrate preparation methods that are highly promising in terms of both signal enhancement and reproducibility with high benefit-cost ratio.

Supported metallic nanoparticles (NPs) have recently become a promising candidate for the degradation of environmental pollutants. In this work, we report the fabrication of highly catalytically active, surfactant-free interconnected Au nanostructures directly on filter paper. The width of nanostructures is only a few nanometers and they contain high concentration of low-coordination atoms. Our approach involves the immersion and evaporation of Au precursor solution on the filter paper and its subsequent reduction. This approach generates filter paper-based gold substrates (FPGS) not only in a simple and straightforward fashion that can be executed in a matter of minutes but can also be easily scaled up for large-scale production. Owing to their surfactant-free nature, FPGS are well suited for the degradation of both cationic and anionic dyes by providing large unhindered surface for dye adsorption. The FPGS were able to completely degrade methyl orange (MO) and methylene blue (MB) dyes and also showed excellent reusability and stability.

We report a facile single-step method for the room-temperature synthesis of gold nanoparticles (Au-NPs) coated with cationic surfactant, Cetyltrimethylammoniumbromide (CTAB). Au-NPs were synthesized simply by reducing HAuCl4 by ascorbic acid in a high pH reaction medium. The size and yield of the synthesized Au-NPs depended on the CTAB concentration in the solution. It was observed that the lower concentrations of CTAB led to the synthesis of Au-NPs of smaller size. Furthermore, we have seen that the conversion of Au precursor into Au-NPs was more efficient for samples prepared with lower concentrations of CTAB. However, NPs synthesized with CTAB concentrations below the second critical micelle concentration (cmc) showed high size dispersion. The CTAB coated Au-NPs exhibited excellent surface enhanced Raman scattering (SERS) and catalytic properties. In particular, they showed great potential for the catalytic degradation of both azo and non-azo dyes, and trace detection of both fluorescent and non-fluorescent molecules.

Simple chemical co-precipitation method has been employed to synthesize two dimensional copper (Cu) doped tungsten oxide (WO3) nanoplates. A numbers of characterization techniques have been used to investigate their structural, optical and biocompatible anti cancer properties. The XRD results have confirmed the monoclinic crystal structure of WO3 nanoplates, and also successful doping of Cu ions into the WO3 crystal lattice. The presence of functional groups and chemical bonding have been verified through FTIR and Raman spectroscopy. The SEM images demonstrate that both undoped and Cu doped WO3 samples have squares plate like morphology. The EDX spectra confirm the presence of Cu, W and O ions. Diffuse reflectance spectroscopy (DRS) analysis has revealed a substantial red-shift in the absorption edge and a decrease in the band gap energy of nanoplates with Cu doping. Photoluminescence spectroscopy has been used to study the presence of defects like oxygen vacancies. Furthermore, the differential cytotoxic properties of Cu doped WO3 samples have been evaluated against human breast (MCF-7) and liver (Hep-2) cancer cells with ectocervical epithelial (HECE) healthy cells. The present findings confirm that the Cu doped WO3 nanoplates can be used as an efficient biocompatible anti cancer agent.

We have devised a simple seedless, high-yield protocol for the synthesis of gold nanoworms (NWs). NWs were grown simply by reducing HAuCl 4 with ascorbic acid (AA) in the presence of cetyltrimethylammonium bromide (CTAB) and NaOH. The nucleation was induced by the addition of AgNO 3 in the reaction medium. We have observed that an optimum range of NaOH concentration is essential for NW formation. Furthermore, we have seen that the synthesis of gold NWs is very sensitive to the supplier of the CTAB used for the synthesis. With the same recipe, CTAB from different suppliers gave different yield and lengths of NWs. The NWs show excellent potential for surface enhanced Raman spectroscopy (SERS).

This report is on the synthesis of Sn doped WO3 nanoplates by a facile co-precipitation method. Various characterization tools have been employed to study the effect of Sn doping on the structural, optical and dielectric properties of WO3 nanoplates. The successful incorporation of dopant ions in the monoclinic structure of WO3 has been verified by XRD, EDX, FTIR and Raman spectroscopy. It has been observed that there is a broadening in optical band gap for doped samples due to the band filling effects caused by the crystal defects. Furthermore, the frequency dependent electrical properties of WO3 nanoplates are found to be significantly tuned by Sn doping. Most importantly, this is one of the initial reports that Sn doped WO3 nanoplates are an excellent candidate for anticancer applications. The anticancer activity of WO3 nanoplates against MCF-7 cancer cells is increased with Sn doping which is attributed to several factors such as particle size, defects density and reactive oxygen species (ROS) production.

Surfactant-free silver nanoparticles (NPs) were synthesized by ablating a silver target placed in pure water by nanosecond pulsed Nd:YAG laser beam. UV–Vis spectroscopy and scanning electron microscopy (SEM) were employed to characterize the NPs. The diagnostics revealed that the NPs concentration in liquid linearly increases with the laser pulse energy up to 40 mJ and then starts to decrease with further increase of the laser energy. The diameter of NPs ranges from approximately 20 to 100 nm, with the average diameter of 50 nm. The average diameter of NPs does not seem to vary with the laser pulse energy. The surfactant-free Ag NPs were used to degrade methylene blue (MB) and methyl orange (MO) dyes. The surfactant-free Ag NPs showed high catalytic activity and completely degraded MB and MO. Furthermore, the SERS study of Rhodamine 6G using Ag NPs as a substrate resulted in significant Raman enhancement. In order to demonstrate that the surfactant-free nature of the Ag NPs is responsible for their better catalytic ability, a cationic surfactant is attached to the Ag NPs, which resulted in substantial reduction of its catalytic ability. These results confirm that the better catalytic ability and Surface Enhanced Raman Spectroscopy (SERS) signal enhancement is primarily due to the surfactant-free nature of the Ag NPs synthesized by pulse laser ablation.

Fabrication of platforms for efficient Raman and fluorescence enhancement is very attractive for plasmon-based molecular sensing. For superior sensitivity, the plasmonic constituents of such platforms should be effective electromagnetic field enhancers. Furthermore, nanoparticles having plasmon peak in the spectral range of therapeutic window are superior for biomedical application. Herein, we show that worm-shaped Au nanoparticles can be used for simultaneous surface enhanced Raman scattering (SERS) and metal enhanced fluorescence (MEF). Gold nanoworms (Au-NWs) with an intense plasmon absorption in the therapeutic window were synthesized using a facile single-stepped recipe. Electron microscopy imaging revealed that Au-NWs have a non-uniform surface. Owing to their special morphology, three distinct plasmon bands were seen in the experimental spectrum. The presence of three plasmon peaks was also verified by finite element based simulations. The simulation results further show that Au-NWs can provide intense near field enhancement for multiple excitation wavelengths. As a proof of concept, we have used Au-NWs based platforms for simultaneous enhancement of fluorescence and Raman signal of rhodamine 6G (R6G) dye molecule.

We describe the synthesis of PZT nanoparticles using a hydrothermal method. We demonstrate that the morphology of the PZT nanoparticles can be controlled by tuning the concentration of the mineraliser, KOH. The experimental results show that for low concentrations of KOH (2 M) the reaction kinetics favor a high yield of spherical particles with an average diameter of 5.3 µm. However, higher concentrations of KOH lead to the formation of branched nanostructures containing up to 8 µm long nanorods. TEM and XRD investigations provide evidence that the branches are single crystalline. The influence of KOH concentration on polymer induced particle formation is discussed.

We report a seedless protocol based on the oriented attachment of nanoparticles for the synthesis of Au nanoworms (NWs). NWs are grown by reducing HAuCl4 with ascorbic acid (AA) in high pH reaction medium and in the presence of growth directional agents, cetyltrimethylammonium bromide (CTAB) and AgNO3. Although we have used the same reducing and growth directional agents as typically used for the synthesis of Au nanorods, the growth mechanism of NWs is markedly different from that of nanorods. Instead of the anisotropic growth of seed particles, the NWs grow through oriented attachment of nanoparticles. By varying different reaction parameters we have seen that the length of NWs can be controlled from tens of nanometers to a micrometer. Furthermore, the aspect ratio (AR) can be tuned from 2 to 30. This is almost the whole range of AR and length for Au nanorods so far achieved with seed-mediated multiple step synthesis protocols.

Seed-mediated methods are widely followed for the synthesis of Au nanorods (NRs). However, mostly dilute concentrations of the Au precursor (HAuCl4) are used in the growth solution, which leads to a low final concentration of NRs. Attempts of increasing the concentration of NRs by simply increasing the concentration of HAuCl4, other reagents in the growth solution and seeds lead to a faster growth kinetics which is not favourable for NR growth. Herein, we demonstrate that the increase in growth kinetics for high concentrations of reagents in growth solution can be neutralised by decreasing the pH of the solution. The synthesis of the NRs can be scaled up by using higher concentrations of reagents and adding an optimum concentration of HCl in the growth solution. The concentration of HAuCl4 in the growth solution can be increased up to 5 mM, and 10–20 times more NRs can be synthesised for the same reaction volume compared to that of the conventional seed-mediated method. We have also noticed that a cetyltrimethylammonium bromide (CTAB)-to-HAuCl4 molar ratio of 50 is sufficient for obtaining high yield of NRs.

For the entire range of spheroidal noble metal nanoparticles, including spheres, rods, and disks, the optical properties are investigated as a function of their geometry and size by means of extinction spectra obtained by numerical methods. For spherical silver and gold particles, Mie theory is used to identify multipole plasmon resonances up to hexadecupole order as a function of particle radius. With increasing particle dimensions, higher-order multipole modes become more important. Moreover, the similarity in dielectric functions of silver and gold gives rise to almost identical optical properties at wavelengths exceeding 700 nm. For oblate and prolate nanoparticles, the dipole, quadrupole, and octupole plasmon resonances for aspect ratios up to 8 are considered. As with the nanospheres, extinction spectra for small nanodisks and nanorods are in agreement with the quasi-static approximation, whereas for larger dimensions, a red shift is observed. The plasmon peak positions are analyzed in terms of a ma...

Surface enhanced Raman scattering (SERS) is an excellent technique for detecting the trace concentration of targets. However, SERS based detection of fluorescent targets is tricky as the fluorescence signal can severely interfere with the SERS signal upon resonance excitation. This can limit the trace detection capability of SERS. Herein, the authors demonstrate that Au nanostars (NSs) are excellent for fluorescence quenching and trace detection by SERS. The NS based SERS substrate was used to measure the R6G concentrations as low as 10 pM upon laser excitation that are in resonance with the fluorescence absorption. The authors used a simple seed-mediated and surfactant assisted method to synthesize AuNSs in a wide size range. The size of the NSs can be tuned from 55 nm to about 1 μm simply by varying the seed to HAuCl4 ratio in the growth solution.

Fabrication of non-functionalized gold nanoparticles is interesting owing to their potential applications in sensing and biomedicine. We report on the synthesis of surfactant-free gold nanoparticles (AuNPs) by Plasma-Liquid Interaction (PLI) technique, using micro-atmospheric pressure D.C. plasma. The effects of discharge parameters, such as discharge current, precursor concentration and gas flow rates on the structure and morphology of AuNPs have been investigated. Optical Emission Spectroscopy (OES) was employed to estimate the UV radiation intensity and OH radical density. Scanning electron microscopy (SEM) and ultraviolet-visible (UV-Vis) optical spectroscopy were employed to study the morphology and structure of AuNPs. The normalized intensities of UV radiation and OH radical density found to increase with increase in discharge current. We observed that the particle size can be tuned by controlling any of the following parameters: intensity of the UV radiation, OH radical density, and concentration of the Au precursor. Interestingly, we found that addition of 1% Ar in the feedstock gas results in formation of relatively uniform size distribution of nanoparticles. The surfactant-free AuNPs, due to their bare-surface, exhibit excellent surface-enhanced Raman scattering (SERS) properties. The SERS study of Rhodamine 6G using AuNPs as substrates, shows significant Raman enhancement and fluorescence quenching, which makes our technique a potentially powerful route to detection of trace amounts of dangerous explosives and other materials.

Seed mediated growth assisted by surfactant is used to grow nanoparticles. The choice of seed particle, i.e. single crystal or multi-twinned, enables growth of nanorods and multi-branched nanoparticles. The latter are generally referred to as nanostars. The shape and size of the nanoparticles can be tuned by varying the solution composition and relative reactant concentrations. The optical properties of nanorods and nanostars are described and discussed in relation to their shape. For nanorods, the aspect ratio is a key parameter determining the plasmon resonance, while the absolute size has a much less pronounced effect. With the nanostars, the length of the branches turns out to be relevant. The optical properties of the multi-branched nanoparticles are discussed in terms of plasmon hybridization

We present the dipole-directed assembly of nickel-coated gold nanorods into nanorings and nanowires. We used two different coating methods to synthesise these core-shell superstructures. Surprisingly, the two coating methods lead to very different kinds of dipole directed assembly. We show that the resultant dipole assembly is very sensitive to the reaction conditions and can be tuned to obtain core-shell nanochains, nanorings, and nanowires. In addition to the presented experimental work, cluster moving Monte Carlo simulations of a system of core-shell nanorods were carried out. These simulations are based on a small number of magnetic interaction energy terms and do not explicitly deal with steric interactions or van der Waals forces. The simulation results are in line with the obtained experimental results, confirming that the magnetic self-assembly of core-shell nanorods can be described by means of a relatively simple model

Nanowires and nanotubes were synthesized from metals and metal oxides using templated cathodic electrodeposition. With templated electrodeposition, small structures are electrodeposited using a template that is the inverse of the final desired shape. Dielectrophoresis was used for the alignment of the as-formed nanowires and nanotubes between prepatterned electrodes. For reproducible nanowire alignment, a universal set of dielectrophoresis parameters to align any arbitrary nanowire material was determined. The parameters include peak-to-peak potential and frequency, thickness of the silicon oxide layer, grounding of the silicon substrate, and nature of the solvent medium used. It involves applying a field with a frequency >10(5) Hz, an insulating silicon oxide layer with a thickness of 2.5 mu m or more, grounding of the underlying silicon substrate, and the use of a solvent medium with a low dielectric constant. In our experiments, we obtained good results by using a peak-to-peak potential of 2.1 V at a frequency of 1.2 x 10(5) Hz. Furthermore, an indirect alignment technique is proposed that prevents short circuiting of nanowires after contacting both electrodes. After alignment, a considerably lower resistivity was found for ZnO nanowires made by templated electrodeposition (2.2-3.4 x 10(-3) Omega m) compared to ZnO nanorods synthesized by electrodeposition (10 Omega m) or molecular beam epitaxy (MBE) (500 Omega m). (C) 2010 Elsevier Inc. All rights reserved.

We demonstrate the synthesis of novel core–shell nanoring superstructures consisting of non-dipolar Au nanorods coated with magnetic Ni shells. We show that the magnetic dipole induced self-assembly can be tuned by selective reduction of Ni on gold nanorods. Superstructures range from nanorings containing physically separated particles to solid rigid nanorings.

We have studied the alignment of colloidal gold nanorods, deposited from solution onto well-defined substrates in the presence of an AC electric field generated by micrometer spaced electrodes. The field strengths employed in our experiments are sufficiently large to overcome Brownian motion and induce accumulation and alignment of the nanorods in the region near the electrodes with their long axis parallel to the field. However, despite the large fields, we find that the degree of alignment is considerably smaller than what was previously reported for field-induced nanorod alignment in suspension. We show that hydrodynamic interactions and capillary effects during drying, as well as friction of nanorods on the substrate surface, to not play a major role. The limited alignment of nanorods is ascribed to the different experimental configuration and the correspondingly larger density of nanorods. The mutual interactions of nanorods give rise to a disturbance of the local electric field and therewith their orientation. For sufficiently large field strengths, these interactions lead to the formation of nanorod chains that ultimately bridge the electrode gap. Furthermore, for small electrode spacing, the nanorods accumulate on the electrode surface, and the screening of their mutual interactions results into considerably improved alignment.

The controlled patterning of anisotropic gold nanoparticles is of crucial importance for many applications related to their optical properties. In this paper, we report that gold nanorods prepared by a seed-mediated synthesis protocol (without any further functionalization) can be selectively deposited on hydrophilic parts of hydrophobic-hydrophilic contrast patterned substrates. We have seen that, when nanorods with lengths much smaller than the width of the hydrophilic stripe are used, they disperse on these stripes with random orientation and tunable uniform particle separation. However, for nanorods having lengths comparable to the width of the hydrophilic stripes, confinement-induced alignment occurs. We observe that different interactions governing the assembly forces can be modulated by controlling the concentration of assembling nanorods and the width of the hydrophilic stripes, leading to markedly different degrees of alignment. Our strategy can be replicated for other anisotropic nanoparticles to produce well-controlled patterning of these nanoentities on surfaces.

We demonstrate a simple and versatile way to achieve high yield synthesis of shape- and size-controlled multi-branched gold nanoparticles (MBNPs). Control over the shape of the MBNPs was achieved by varying the ratio of gold to the mild reducing agent ascorbic acid, using a seed-mediated growth approach. Higher ascorbate concentrations resulted in the smoothing of branches, leading to the yield of relatively more isotropic particles. Furthermore, we found that using much higher silver concentrations in the growth solution resulted in the formation of rod-shaped micro-features together with MBNPs; we postulate them to be cetyltrimethyl ammonium silver bromide crystals. The as-prepared MBNPs show interesting tunable optical properties that are strongly influenced by the particle shape. The results are discussed in terms of plasmon coupling between the core and branches of the MBNPs.